Aqueous alkali metal hydroxide solution is used for removal of various toxicants like sulphides, merceptans, amines, naphthenic acids phenols etc from gaseous and hydrocarbon streams in oil refinery processes. Once these contaminants come in contact and react with caustic solution, it cannot be further utilized and is known as spent caustic. The typical composition of spent caustic may contain about 3-12% of the NaOH along with significant quantities of toxic compounds like sulphides, mercaptans, amines, naphthenic acids, phenols and their derivatives, hydrocarbons and few other inorganic and organic compounds. Owing to presence of these contaminants and high salinity and high pH, spent caustics are most difficult of all industrial wastes to dispose properly. Spent caustic is disposed off by very expensive and environmentally reactive methods such as high dilutions and then treatment at ETP, deep well injections, incineration, wet air oxidation, humid hydrogen peroxide oxidation etc.
In addition to above, waste streams like flue gas containing CO2 and other air pollutants and desalter washing which contains high concentration of salt are also produced in refinery operation which poised negative impact on environment and need to be treated.
US 2004/0040671A1 describes a method in which SC effluent is supplied to a submerged combustion gas evaporator in which hot combustion gas containing CO2 is injected into caustic liquid to concentrate the liquid and convert a hydroxide constituent to a carbonates which is then separated from waste stream.
U.S. Pat. No. 5,244,756 describes a process which introduces refinery gas containing CO2 and hydrogen sulfide into a sodium hydroxide solution to convert the carbon dioxide to sodium carbonates.
U.S. Pat. No. 7,713,399 B2 described a method for treatment of spent caustic where CO2 is used and aqueous sodium sulfate, sodium carbonate and sodium chloride is obtained.
EP2354099 discloses a process for conversion of divalent cations present in waste brine into useful carbonates, using waste gas stream containing CO2 in the presence of halotolerant microorganisms exhibiting carbonic anhydrase activity.
U.S. Pat. Nos. 8,480,796, 8,329,498, 8,329,460, US 20130224842, US20120107899, CA 2813640 A1, WO2008095057A2 disclose the application of carbonic anhydrase for CO2 capture. All methods discussed therein use costly buffers for enzyme activity, which is not required in present invention. Moreover, none of the prior arts have disclosed use of carbonic anhydrase which can work at pH>13 and salinity >4% and also none of the prior arts have used this enzyme for treatment of waste streams like spent caustic along with CO2.
The present invention addresses the above problems which involves the treatment of refinery waste into industrially useful products by an environment friendly method using a carbonic anhydrase enzyme at high pH (>10) and salinity (0.1-10%) and converting waste refinery streams comprising of spent caustic, brine streams and flue gases containing CO2 into useful products such as nano-sized carbonates. The present invention also addresses the problem of disposal of residual liquid phase by treatment with a consortia of specific microbes.